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How to read this old French speed gauge?
Vitesses au Sol is ground speed, and Vitesses Vrais is true air speed, but

what is the backward scale 450 to 750 for?
how does one read altitude?
the bendy wire looks like it's probably related to the true airspeed markings, but how?
why did this elegant design not catch on?

In short, how is it to be read as its arms and displays rotate?

Source

Source
This second one is similar but oddly the spiral is reversed. A US Patent for this instrument (1919) is available and may provide clues to those attuned to patent descriptions.

Wow. That is a beautiful instrument.

Bendy wire of the top one seems the wrong way around. Bottom one makes way more sense. Poor "restauration" I'd guess?

Regarding why it didn't catch on, there might be a clue in a page @mins links to, which describes the device as 17cm in diameter. Huge!

Adding to @Camille's good answer.

This instrument combines different airspeed and altitude indications:

Airspeeds in the top half use two scales: The outer vitesses au sol scale and a scale with curved marks vitesses vraies close to the center. Both scales are likely km/h considering the era, less likely knots. (The actual unit has no influence on the instrument).

Altitudes in the bottom half: The outer pressions barométriques, static pressure in mmHg, a fixed scale, and inner altitudes, a moving scale in meters. This scale can be rotated using a screw or a knob. Altitude 0 m would be aligned with the local QNH (760 mmHg for the standard pressure). This is similar to the setting of a modern altimeter using the Kollsman window.

My understanding is airspeed is measured using the dynamic pressure from a pitot tube, and altitude is measured using the static pressure from a static port, thus the instrument basically combines an airspeed indicator with an altimeter. However the instrument is also a mechanical calculator producing a correction factor and applying it to the dynamic pressure from the pitot to get the true airspeed (TAS).
It was invented by Maurice Dugit-Gros ca. 1919 and manufactured by SIAM. The patent applies to a broader scope: A method and apparatus for the mechanical calculation and display of functions depending on non-linear factors. This instrument is indeed an application of this patent to calculate and display the true airspeed.
About TAS and the upper outer scale vitesses au sol
Airspeed value is often a mapping of the dynamic pressure value measured with a pitot tube. Airspeed "calculated" this way is the indicated airspeed (IAS). The pitot pressure is however subject to several errors, the largest being related to its placement on the aircraft and the aircraft own orientation relative to the airflow.
Other airspeeds are derived from IAS: Removing pitot errors from IAS gives the calibrated airspeed (CAS); Removing errors associated with air compressibility from CAS gives the equivalent airspeed (EAS); Removing errors associated with the variation of air density with altitude from EAS gives the true airspeed (TAS). See airspeed calculator and conversion.
Ground speed (GS) and airspeeds are related by wind speed. GS is the vector sum of the aircraft speed in the air mass and the air mass speed relative to the ground (i.e. wind). In no wind condition, the two speeds are equal (GS is still affected by the errors associated with the chosen airspeed).
In the instrument shown two types of speeds are displayed: vitesse vraie and vitesse au sol, which are literally translated as true speed and ground speed.

The former is likely TAS. The red straight hand of the top half sweeps both the TAS scale and the outer scale. This hand is driven by the pitot dynamic pressure, but some correction is applied in the mechanics behind the instrument plate, also using a scale with curved marks and a spiral hand. This complex setting is required to perform the mechanical calculation of the TAS which is a non-linear function.

The latter vitesse au sol (ground speed) is more difficult to interpret: Displaying GS appears curious for this era. GS calculation implies wind speed knowledge, or another mean for speed measurement, not based on pitot data.
Both @Kenn Sebesta and @Sanchises spotted this strangeness, as well as @Camille Goudeseune in his answer. Warned by their inputs, I've reworked my answer. I'm also convinced the scale is not a GS scale in spite of the name and I'm also inclined to think it should be IAS, the information given by a basic airspeed indicator.
The reason the inventor used GS instead of IAS might be corresponding definitions didn't exist ca. 1920, the full model of airspeed measurement being still to establish. This should be digged further, but IAS appears much more plausible than GS.

Bottom altimeter
The white hand with a double black head sweeps over a scale of atmospheric pressures (pressions barométriques), it is driven by the static pressure from a static port.
The leftmost indication is 780 mmHg. The mean sea level (MSL) pressure is 760 mmHg and pressure continues to decrease with altitude. (On the picture I used below the 750 mark is slightly offset.)
A corresponding altitude scale is aligned with the static pressure scale. The altitude mark 0 m must be aligned with the pressure at MSL (the QNH) or 760 mmHg (the QNE) if the QNH is unknown.
TAS reading
The white altimeter hand has a red spiral extension which sweeps the TAS scale of the upper half. As we saw, TAS must correct for density variation with static pressure. So TAS must be mechanically calculated using both the dynamic pressure (red straight hand of the top half) and static pressure (red spiral hand fixed to the altimeter hand).
The TAS value is read at the intersection of these two red hands.
Example
Let's take an example where the straight red hand indicates 160 on the IAS (outer) scale, and the white hand indicates 1524 m (5000 ft) on the bottom altitude scale:

Readings are:

IAS = 160 (likely km/h), under the orange straight hand.

TAS = 170 (same unit as IAS), at the intersection of the orange and the blue spiral hands, over the scale with curved marks.

Static pressure = 630 mmHg (840 hPa, 24.8 inHg), blue hand over outer scale.

QNH = 749 mmHg (999 hPa, 29.49 inHg). Set with the knob.

Altitude = 1524 m, bottom blue hand over inner scale.

With the white altimeter hand in the original position of the photo (100 m) and the IAS still equal to 160, the TAS reading is different. The intersection of the orange hand and the red spiral hand gives a TAS of 160. The equality TAS and IAS results from the fact compressibility and density errors are about null near MSL.
You can check other IAS (orange lines) with the IAS-TAS mapping for 5 000 ft in this question: What is the relation between IAS and TAS at a constant altitude? It's actually the CAS-TAS mapping, but we can assume IAS ≈ CAS given the low IAS (low compressibility) and the precision of the instrument.

Spiral hand direction

This second one is similar but oddly the spiral is reversed.

On the first photo, the spiral hand has been assembled the wrong way: When altitude increases (CCW rotation) TAS must also increase. As it is mounted, TAS decreases. On the second photo, the spiral hand is in the correct position.
Forgotten instrument

why did this elegant design not catch on?

Likely because IAS is sufficient for flying at moderate speeds, and there are dedicated electronic devices for TAS and Mach for faster aircraft.
Such instruments were not uncommon before the advent of analog computers. It is little-known but France has been a pioneer for aviation and aviation instruments. The basic airspeed indicator (ASI) was invented in France  (locally this instrument is known by the name of its inventor, a Badin).
SIAM (which manufactured the Dugit-Gros instrument) and SFIM were specialized in manufacturing complex instruments. Some, military oriented, were later used on the Dassault Mirage series (e.g. the Bézu 3D attitude indicator, another Maurice), other, civil-oriented, were later used on Concorde.
Regarding information about older instruments, there is a historical document from Jean Carpentier, Un demi-siècle d'aéronautique en France, unfortunately only in French.

I'm not sure that <> is in fact "ground speed" here. Looking at https://www.comptoir-aviation.com/le_comptoir_de_laviation/2017/images/171212_indicateur-Dugit_02.jpg, it's pretty clear that for a given <> true airspeed varies as a function of the spiral dial. This would point toward this being IAS, and perhaps <> might have meant in 1919 the speed of the vehicle at sea level on an STP day. (And FWIW, the patent doesn't answer this question.)

Thanks, that's a very helpful diagram. Still, the outer dial doesn't look like windless ground speed, which would equal TAS. (Since TAS is defined as the speed moving relative to the air mass, and if the air mass isn't moving than that equates the (modern?) definition of ground speed). Perhaps there's a typo when you say Ground speed (GS) without wind (TAS at mean seal level (MSL))?

I agree with Kenn. I don't think "Vitesse au sol" means Ground Speed as known in English, but something like "equivalent airspeed as if you were flying close to the ground" - known as Equivalent Airspeed in English.

@mins I think we are, rereading your answer. I think the confusion comes from your first bulleted point which is ambiguous and is being interpreted as current TAS which is not TAS as if the plane were at ground level. I have made an edit which I think reflects this better, feel free to revert if this isn't to your liking.

@KennSebesta and Sanchises: Your inputs are meaningful, I reworked my answer to take your suggestions into account, I opted for IAS, but I agree it could be EAS too. Thanks to both of you.

@mins great work, much better than my version!

For a start:
The bottom half's scale 750 ... 450 is barometric pressure in mm Hg.  Twirl the thumbscrew at right to calibrate the rotating altitude scale 0 ... 4000 (metres) to match the current mm Hg at ground level.  Twirl it until 0 feet matches current conditions.  Then the needle that shows barometric pressure gives you altitude.  That may be the needle reading 450 mm Hg in this photo.
The other needle then reads roughly zero airspeed.  The spiral from its tip can't match true to indicated airspeed (how would 40 match 260?).  The airspeed needle then points to both the true and (approximate) indicated airspeeds.  What are the spiral's tips really attached to, the needle(s), or the faceplate?

I bet the units are metric.


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Certification of Aircraft Phone/Tablet Mounts/Holders
I would like to know, whether it is required for such items to be certified for application on genaral and/or commercial aircraft:
https://www.rammount.com/part/RAM-B-166-MP1U
https://mgfproducts.com/products/sport-mount-flex-suction?variant=10434660040819
In what countries is it necessary?
Is it compulsory or not for them to be certified?
I would be gratuful for any help!)

For general aviation no certification is required, neither is certification required for at least some commercial operations like flight training, at least for the regulatory bodies I'm familiar with (FAA and EASA). I've seen Ram and Garmin mounts being used on some smaller passenger and cargo operations as well, usually GPS and tablets in older prop airplanes like the DC3, Beechcraft twins and the like.
As for carrier airline operations I am not sure.

For items like that are that not permanently attached, certification on the aircraft itself is not an issue because from the airplane's perspective, the item doesn't exist (it would be no different than attaching a pen to something with chewing gum or scotch tape).
However, a local authority may have a regulation against temporarily attached devices for whatever reason, or a manufacturer may have a restrictive policy that affects warranty (like say, a manufacturer may have a policy that says suction cup devices void warranty on new aircraft windshields or windows to which they are attached).

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What does the word "boot" mean?
I found this word "boot" in one book. The chapter was connected with ICING operations, and I found this word with little explanation:

A tube bonded to a surface, e.g. wing edge. When pressurized with fluid, it breaks up ice.

Please explain what is it?
What aircraft equipped with it?
Photos would be nice.

The word "boot" applies in two ways here: one because it covers the leading  edge (it even was called "overshoe" at some point) and second because it "boots" (ejects) the ice away.

This question refers to de-icing equipment, but for the sake of completeness, boot can also refer to wheel boots, an aerodynamic covering sometimes placed around wheels on fixed landing gear aircraft http://aircraftpaintprotection.com/image/cache/Products/lopresti_iceskates_paintprotection-250x250.jpg another commonly used term is wheel pants.

Versalog already told the core of the thing. Most prop aircraft which are approved for flights into known icing conditions have such boots to remove the ice from the wings leading edge.
Those boots are either inflated manually whenever you need them (by the press of a button of course), at regular intervals or automatically whenever ice is detected.
Check this picture, the black thing is the boot on a Dash 8's wing:

Link

Is it possible boot to be another color?
And why black?

@wiaim: It's rubber, just like tires.

Thank you, but why rubber to increase heat transfer?

Rubber is flexible. Rubber is black because of added carbon, which drastically increases the durability and strength of the rubber. It's also good at scattering/absorbing UV, which is much more intense at altitude and would rapidly destroy the rubber.

@wiaim It's not to do with heat. When it activates it inflates sort of like a balloon and causes the ice to break off and blow away. Anti-ice by heating is a different type of system that doesn't involve a boot

Some jets—some of the Citation series for example—also have such boots.

Find a picture of any WWII bomber or any modern Cessna Caravan and there should be a black stripe along the leading edge of the wing and tail surfaces. That is termed a boot.

(Image source)
Inflating it slightly will make the ice break and fall off the wing.

Hello Versalog, welcome to aviation.stackexchange.com! It would be great if you could find a free (non-copyrighted) image of an aircraft with de-icing boots to improve your answer.

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Is the word "retard" really spoken when some planes land?
I've been watching videos of plane landings, and I've noticed a certain word being said while some types of planes are very close to the ground. It sounds a lot like the word "retard," which surprises me since that word is often considered offensive. Is that really the word being said? If so, why is it being used in this context?

A simple search will show it's a common term in aviation: https://aviation.stackexchange.com/search?q=retard - the kids' slur is much more recent and unrelated.

@Therac They're not completely unrelated; they share a common root. The kid's slur comes from the no-longer-used medical term. In both cases (as well as when adjusting engine timings), it's being used as an antonym for 'to advance'; i.e. slow down or pull back.

There's lots of words that have a technical meaning and are also rude if used in a different context.

@SomeoneSomewhereSupportsMonica Common root, but one's a noun contraction from "mentally retarded" and the other's a verb. So their use is distinct both gramatically and phonetically.

You can tell someone's relatively young by how worked up they get over benign words like retard. Going to great lengths even to try avoiding saying it themselves.

It's a quite common term in physics: Retarded potential retarded time

Outside of aviation, you can also find road vehicles equipped with an indicator light assessing the mental capacity of the driver: "Retard on" dash light, what does it mean?

@SnakeDoc Yep, a person's age connects to where they're anchored in the history of "euphemism inflation" ... just like when people say "I remember when gas was 99 cents a gallon!"

The emphasis may be important here; when used as a verb, the emphasis is on the second syllable while when used as a (derogatory) noun it’s on the first.

@SnakeDoc Seems that depends entirely on your definition of the word "benign" and how widespread that definition is. The word "nigger" was considered benign by many just a few decades ago, and those people would have been wrong. For the record, I don't have a problem with "retard" because I've always used it and my social circles aren't generally the kind who would be offended by it, but I also have the tact not to use it among others because I know most nowadays wouldn't consider it benign, and I also think it's idiocy to pretend that old people are immune to being offended by words.

How in the world did this become a Hot Network Question on Stack Exchange?

It's surprising to see this question being so popular. Even today's young people must surely know there is a difference between calling someone 'retarded' as an insult, and 'mental retardation' as a medical diagnosis (for example). I refuse to believe the depressing alternative.

This reminds me of the computer user who made a furious call to the support line complaining that his computer was accusing him of being an invalid. The error message in question was "invalid user input." Words have multiple meanings, and technical jargon introduces still more meetings for words.

@HashimAziz, is it inherent to the word that it's not ok, or does it have something to do with the context it was used in, or the general state of race issues at the time?

@keshlam I wonder when people will start complaining about a dialogue with an "Ok" and a "Cancel" button.

@keshlam similarly, I have heard stories of complaints by patients against physicians for describing the patient as an "SOB". This is an an abbreviation for the common insult "Son Of a Bitch" but is also a medical term for "Shortness Of Breath".

What you're hearing is an automated callout on Airbus aircraft directing the pilots to retard the power to idle. It's a verb, used as an imperative.
If it were being used as a noun, or more specifically, if it was being used as an epithet for someone, then that would be offensive, but that's not the case here.

The two uses are also pronounced differently; the pejorative sense places stress on the first syllable, while the verb form places stress on the second (think "fire retardant").

The word means delay or hold back.  In this context it means to bring the engine power back.  In the pejorative sense it is a claim that a person is developmentally delayed.

@Anerdw Only that the Airbus callout places stress on the first syllable.

@AndreKR Wow, that is an...interesting decision. Well done, Airbus. I guess.

@Anerdw It might be synthesized speech instead of a recording, the text-to-speech system wouldn't know the context.

Or it's simply a direct translation of the word found in most romance languages, which literally have no other synonym for the concept of "delay" (unless you want to get fancy with the equivalent likes of "postpone" or "procrastinate")

ONLY on Airbus? I've heard this on many landing videos.

@MikeWaters Perhaps share a video link from a non-Airbus airliner?

Airbus uses the word "retard" for reducing the throttle on landing, and it's spoken by the proximity system.
Example from brake testing documentation:

Engines must be at idle at touchdown, with the throttles chopped to idle at the “RETARD” automatic call out.  At touchdown, manual braking is immediately applied to maximum pedal deflection

In some radio altimeter errors:

Heavy landing in AUTOLAND and a continuous RETARD call out.

Besides cutting the throttle, at what point would  reverse thrust be used? (I know, this probably deserves its own question. ;-)

You have actually answered this backwards. Airbus uses the word 'retard' to increase how much the engine is throttled, which means to reduce the power output. And the confusion with words like throttle is why a plain and unambiguous word like retard is used - it has only one interpretation, which is to reduce the power produced by the engines.

@fabspro Ah, there we go with ambiguous words again. Is “full throttle” a fast engine or one with low power? If “full throttle” and “throttle up” are equal, then retarding the throttle reduces it.

Throttling an engine literally means reducing the air intake, reducing the throttle is ambiguous. I was simply saying that your answer would have been clear if you had used a phrase like "reduce the engine power". I agree with you that "full throttle" and "throttle up" are ambiguous phrases as well.

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What does the term "tanker" mean when used in regards to a passenger airliner?
In a comment on this answer:

Airlines will tanker fuel if it's substantially cheaper at another airport

I've wondered what that term meant before, and now this comment leads me to think that airliners will carry more fuel than what's required for a trip, because carrying the fuel might be cheaper than buying it at the next airport.
Can someone provide more detail on why and when an airline might do this, the calculations that are involved, and any other interesting information on this topic?

Back in the 1990s, tankering was also an option to avoid refueling at outlying fields where there were fuel contamination worries or to avoid delays due to having to use over-the-wing refueling. The rough rule of thumb for 747-100/200 aircraft was that if you tankered fuel, it would take 20% of that fuel to do it. In other words, if you wanted an extra 40,000 lbs available at your destination, you had to load 50,000 lbs.

@mins I've never been aware of that distinction. Perhaps meanings have changed since I retired, a lot of things have. All I can offer is that at the two 747 carriers I flew for, in the common parlance between dispatcher and captain, carrying any fuel not required to fly the leg was spoken of as tankering, except of course fuel loaded for ballast purposes.

Since the 1960's and 1970's when political hijackings were a problem, airplanes started to only carry the fuel required for the flight.  This means a typical fuel load will be something on the order of:

taxi fuel at the departure airport
fuel to fly and land at the destination at a given altitude and forecast winds
if an alternate is needed:

fuel to fly to the alternate airport

45 minutes of reserve fuel
perhaps a bit extra for contingency and to keep captains happy (e.g. 5-15 minutes worth of fuel).

For a normal flight this means you'll land with around 45-60 minutes of fuel on board, sometimes less.  You'll fill up for the next leg at this airport while you deplane and enplane passengers.
Fuel at the hub airports is sometimes a bit cheaper due to the fuel contracts the airline can secure when they order massive amounts of fuel every day.  Fuel at outstations that don't see a huge volume of fuel may be quite a bit more expensive.  In this case, and if your weights allow, the airline may elect to load extra fuel that isn't needed.  The purpose of this is so you do not need to buy as much fuel at higher prices at the outstation.
I only saw this happen operationally when we were going to be very light and the outstation fuel was very expensive.  Usually our loads were high enough that we were struggling to get all 50 passengers, bags, and especially jumpseaters on board with the required fuel load.  In that case it would not be economical to take more fuel because we'd have to take fewer passengers.

Was this primarily about hijacks? Less fuel means lower wait which means lower fuel burn, with rather significant impact on the overall operating cost.

@JanHudec: While less fuel can be loaded faster (less wait indeed), I wonder from your comment on fuel consumption, if you didn't mean weight

@BenVoigt, yes, I meant weight. Unfortunately comments cannot be corrected.

Tankering fuel means that an aircraft carries not only the fuel quantity required for the flight to the destination, but also the fuel, or a part of it, required for the next leg of the trip. The aircraft is therefore its own tanker.
Tankering means reduced cost
This practice can be used when the difference in fuel price at departure and destination airports makes the operation less expensive than refueling at destination. The time to refuel can be also a factor. More: The benefits of tankering considering cost index flying and optional refuelling stops.
From the previous article:

Fuel expenses remain one of the major components of an airline operational cost, depending on the aircraft size, it may approach from
20% up to 70% of its total flight cost.

Fuel tankering has been studied and applied by airlines at least since the oil crisis in the 1970 decade. It has become an excellent
fuel saving strategy for airliners proving costs reductions up to 10%.

Example of price difference in Europe:

Difference in fuel prices in Europe, source Eurocontrol
Example of savings by fuel tankering:

Savings by fuel tankering for airports distant by 300 NM. Eurocontrol
How to read: For a round trip between airports distant by 300 NM, if fuel is 20% cheaper at the departure airport, then by performing a full tankering, 45 kg of additional fuel will be needed, but this will save 196 € (8%) for the round trip.
Tankering means also more CO2
Tankering means burning more fuel than strictly required, this can provide an economical advantage, but this operation is usually not neutral for our environment, depending on how fuel is delivered to airports. From the study by Eurocontrol:

[...] it  was  estimated  that  fuel  tankering  could  result  in  a  net
saving  of  265M€  per  year  for  the  airlines.  However,  it  would
generate  286,000  additional  tonnes  of  fuel  burnt  and  901,000
tonnes  of CO2 emissions at ECAC level per year.
This represents about 2,800 round-trips between Paris and New York or
the annual emissions  of  a  European  city  of  100,000  inhabitants.

(ECAC is the European conference for civil aviation)
Hence:

British Airways reviews 'fuel-tankering' over climate concerns
Thomsonfly reduces tankering and CO2 emissions
...

+1 for adding While there is sometimes an economical advantage to burning more fuel than strictly required, this operation is not neutral for our environment.

It basically means they will take on more fuel than necessary because the cost of doing so is cheaper than getting the fuel at the destination airport.

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Why is mid-air refueling worth it? (or is it at all?)
Surely, mid-air refueling is a cool thing, but is it worth it?
It's obvious that the military, for one, has some requirements to keep aircraft airborne so that they can keep patrolling, for example.
However, that is due to the need to keep the aircraft airborne and is not necessarily because the method is cost-effective.
Flying another aircraft carrying lots of fuel just doesn't seem very money/environment-friendly – you waste more fuel needed to power the fuel-carrying aircraft in addition to the fuel you would already need.

Is mid-air refueling cost-effective under any circumstances? Or is it just an emergency practice?

Eco-friendly? Surely not. An important point is that it adds range though and would allow specialized aircraft to reach targets it otherwise wouldn't have. It's a lot more a question of logistics than of money.

Related: How do jet fighters get to war?

Welcome to Aviation.SE! This is an interesting question, but it might be more clear if you specified the economic focus in the title rather than just at the end of the question. In several circumstances (this being one of them) a higher cost solution may well be worthwhile if it's faster.

There is at least one airplane that can only takeoff and do half its initial climb before it needs more fuel. Without midair refueling that plane would be a paperweight.

@casey Would that plane be the SR-71?

@reirab I believe it is. At the very least, that's true of the SR-71 -- that's partly because it leaks fuel at low speeds, though (the fuel tank was designed to be leaky at low speeds and temperatures, because at Mach 3 it would expand and this way there was room for it to do that), and partly because it took off with less than full fuel to reduce weight at takeoff.

@casey I prefer my paperweights to not drip fuel on my papers.

Why would it be inefficient? I guess a large transport aircraft has much better fuel efficiency per wight than a fighter aircraft. I don't have any numbers, but as an educated guess, I'm fairly certain that a fighter has to burn much more fuel than a large transport aircraft to transport (or even take off with) x kg of extra fuel.

Sometimes it's just necessary. Here is one mission that would be impossible without mid-air refueling. You CAN build a plane specifically for the mission that wouldn't require mid-air refueling but the war would have ended by then: https://www.youtube.com/watch?v=PBJ99bIhAVk

Here's a paper on using in flight refueling for commercial long flights to save fuel and engine wear.  http://aerosociety.com/News/Publications/Aero-Journal/Online/388/Operations-and-aircraft-design-towards-greener-civil-aviation-using-airtoair-refuelling. Sorry about the paywall.

In some contexts, the economics depends on how special the aircraft you're refueling is.
Consider  something like an AWACS mission. You have this very complex specialized aircraft, with a large, highly trained, crew in addition to the flight crew. If you didn't refuel that during the mission, it might need to spend a lot of its active hours being unproductive en route to and from the war. You would need more of them (and more crews) to reach any given level of in-theater coverage.
On the other hand with aerial refueling you need some tankers scuttling back and forth with fuel for your sentries. But they are cheaper and simpler to operate, cheaper and simpler to crew, and so forth. So even though you may use slightly more aircraft hours in total, the total  costs can be lower.
This doesn't hold to just the same extent for fighters and bombers, because they have to return to base from time to time to stock up on munitions anyway. There are still "readyness" types of missions it would be relevant for. The price tag for an F-35 seems to be in about the same ballpark as for a KC-767, but a KC-767 can service many F-35s in a single mission, so having the tankers do the scuttling-back-and-forth can still be an economic win.
Also, it's a good thing for a fighter that gets into combat not to be lugging around enough fuel to get all the way home by itself.

One more good point that would make this excellent answer a complete one; very often, bases for fighters cannot always be located within range of the operating theater, due to political or practical concerns. In the first Desert Storm, the USAF operated primarily from Riyadh Air Base in Saudi Arabia and a couple carriers in the Gulf, with a few FABs for A-10s and helicopters further north. The F-15, the U.S's primary deep strike weapon for that campaign, could not get from Riyadh to Baghdad and back without IAR if the mission called for 2000-lb GBU-10s to be delivered to the target.

First to the issue of cost-effectiveness: If fighter or strike aircraft would need to take more fuel with them, they would have to be larger and more fuel-hungry themselves. This is hard to quantify, but with air refuelling you can cap the size at  a much smaller value and still be sure that all future range and endurance requirements can be met.
Next, modern air campaigns are extremely complex. It takes months of planning for a single day of multi-national operations, and aircraft from several countries have to be staged at some distance to then fly their sorties with utmost precision. Air refueling makes it much simpler to do this planning, because it allows to build in waiting times or to use scarce ressources multiple times during a day. Take electronic countermeasures aircraft: Instead of flying back to a remote home base, the aircraft can refuel at an airborne tanker and be back to support the next wave of attacking aircraft.
And whoever said that concerns about their environmental impact have ever influenced the choice of means of the military?

Environmental impact isn't the major consideration but, sure, it's an influence. The British Royal Navy has scrapped its single-hulled tanker ships, for example. And military budgets are under pressure in most places, so burning less fuel is certainly something that's taken into account.

Kind of a quasi-Tsiolkovsky situation; Space rockets have to lift their own fuel, and they don't have the option of in-air refueling, so for each additional pound of payload, you need the additional fuel to get that pound into orbit, plus all the fuel needed to get the additional fuel at least partway. The same ultimately applies to an aircraft, albeit with lower speeds; to increase your range without midair refueling, you need the fuel to go the extra X miles, plus the extra fuel to get back those extra X miles, plus the fuel needed to get and keep that extra fuel in the air.

Do you think the weight of cargo comes into play as well? I'm just considering, why should a fighter or strike aircraft overload itself with fuel? They can just carry more ammo and little fuel (hence, the need to refuel without returning to base).

@iroegbu: Fighter aircraft are space limited. The internal volume is kept to a minimum to improve supersonic performance and keep structural mass low. Everything else is hung on external stores. So you're right in a way, but it is the number of hard points that sets the limit, and not the weight.

It's not a question of saving money, or emergencies. The reason militaries use mid-air refueling is so they can reach anywhere in the world. Aircraft have a limited range, without mid-air refueling they would have to have enough fuel to get to and from their area of operations back to a base. This would limit where military aircraft (or espionage aircraft) could reach, even with bases all over the world.
With mid-air refueling aircraft can fly great distances from their bases over any territory, complete their mission, and get back. It greatly increases a military air force's capability.

There's one other important reason why military aircraft use airborn refueling.  Naval aviation is limited by maximum takeoff weights that are much lower than similar ground based aircraft with access to longer runways can achieve.  As a result, they often take off with fuel tanks only partially filled and then immediately tank afterwards.  At the extreme, for bombing missions with heavy ordinance loads the on board fuel tanks may be almost empty with the majority of fuel being provided after takeoff.

Another way to put it would be to say that if a journey would require refueling, it will be necessary to have someplace along the way to refuel. Establishing such a place on open water, or on another country's territory, is often difficult or impossible compared with establishing such a place in the air. Additionally, ground facilities are much more subject to attack by enemy ground forces than would be a mid-air "fueling depot".

@DanNeely, re naval: But how does the tanker get to be there in the first place? It can't have launched from the carrier itself, under the same restrictions? If you own an airbase close enough to the war for your tanker aircraft to be able to get there, what do you need a carrier on the scene for?

@HenningMakholm the tanker is lifting off with x tons of fuel, instead of x tons of bombs/missiles/etc.  At times in the past the USN has used dedicated tanker aircraft (ex KA-6D ); although currently it's using F18s (2nd para) with multiple external tanks attached to the hard points; typically in a 4 strike to 1 tanker mix.

@HenningMakholm Dedicated tankers also tend to have much greater range than strike aircraft (a side effect of being big slow airliner derivatives, instead of things meant to stand up in combat), and can be shared across friendly air forces. You could well have a tanker based further from the scene than your carrier, and still have it work out better than not using a tanker.

Some countries have political issues with allowing other countries armed aircraft to land and refuel. An unarmed tanker may be a piece of military hardware, but no arms makes for an easy political situation.

I can second @CGCampbell 's explanation from my own service experience in Switzerland: We do not allow armed airplanes (incl. transporter carrying arms or combat personel) to cross through our airspace but tankers, ambulance planes, etc. are fine. The same also goes for Austria, as far as I know, and their airspace cuts right through your path if you need to get from Germany to Afghanistan or something like that.

@DanNeely The Super Hornet's max takeoff weight of 66,000 pounds is also its max catapult weight.  There is no distinction between ship and shore operations.  The Rhino's payload of 34,000 pounds is nearly twice the weight of an empty F16, and about 14,000 pounds greater than its max payload.  Even the F15 can only carry about 6,000 pounds more than the Rhino. Carrier operations do not effect the carrying capacity of this aircraft at all. The Rhino is a pretty impressive bird!

@supercat: "Additionally, ground facilities are much more subject to attack by enemy ground forces" -- and espionage. My understanding is that by policy the stealth bomber didn't used to land in the UK (or outside the US in general), but did attend UK airshows from time to time without landing. It can't make the round-trip without mid-air refuelling. There have been landings in the last few years, so the policy changed eventually.

Cost effective?
Look up the Black Bucks raids, then work out how much that refuelling cost.... Then compare that to the cost to the UK of losing the Falklands Islands.
Those raids were absolutely not possible without midair refuelling. Similarly Cold War patrols and reconnaissance missions. It's not about whether it's cost effective, it's about whether the mission can be undertaken without refuelling.

+1: The military is not a for-profit enterprise. Their job is to provide capabilities that their government needs; money is just a means to that end, not an end in itself.

The British treasury would be better off without the Falklands. All they provide is pride and prickly relations with Argentina, for the cost of £300,000 per family.

Firstly I'd like to see a citation for that £300k/year claim: but the oil around the island is potentially far more lucrative. More importantly though, you're right: Pride. Name me another country which would meekly hand over territory and those citizens resident there? I'd expect my government to protect my home, cost is irrelevant in that case. You don't ignore your people just because they aren't profitable.

In addition to other answers, tankers can provide specialization for their task -- i.e. circling for a long time where no one is shooting at you. Many military aircraft have to massively compromise fuel efficiency and capacity to do their job. Dedicated tankers tend to be variants of airliners; they have lots of carrying capacity for fuel and can be designed with fuel efficiency as a concern. The other aircraft can then optimize for things besides fuel capacity, and don't need to worry so much about being extremely heavy and (for fighters) having unaerodynamic drop tanks.
To use the SR-71 as an example, it couldn't carry much fuel at all and had horrendously inefficient engines (it was a plane that routinely cruised on afterburner) in order to do its job of flying faster and higher than anything else in the sky that wasn't a rocket. You could give it a high fuel capacity, but since it leaked fuel at lower speeds (the fuel tanks only sealed under thermal expansion) there wouldn't be much point, and you'd have to deal with a really heavy plane not optimized for low-speed flight during takeoff. However, the KC-135Q that refueled it did not have to fly at Mach 3, so it could be designed to be able to take off with plenty of fuel. You could not build a plane to do what the SR-71 was supposed to do and be able to operate without tanker support; the designs for "able to take off with all the fuel you'll need" and "able to do your job" conflict, and the best way to resolve it is to split them into separate aircraft. So in that case, it would be far, far more expensive to build something to approximate the same capabilities without a tanker.
(The main reason, though, is what Jon Story said: Midair refueling is used when you have a job to do, not when you're trying to make money through your aviation operations. If you're trying to make money by flying things around, you wouldn't be doing the missions that require mid-air refueling. You only use it when your guiding principle is "here's your job, here's a budget, go and do it," not "go and make money the best way you can.")

The SR-71 wasn't that inefficient, was it? Usually afterburners are inefficient because they're designed for emergency use, not sustained use. But on planes like the SR-71, you can optimize for afterburner use. Besides, " inefficient"  is relative. What is efficient at Mach 3.2?

@MSalters Exactly. There isn't really a way to make an efficient plane to fly at Mach 3.2. So you can fly more fuel on a plane that is efficient (KC-135Q), and then not need to push even more fuel through the sky at Mach 3.2.

I'll build on @HenningMakholm's excellent answer.  Mid-air refueling brings lots of cost-benefits beyond the obvious "make planes go farther".  First, tankers can provide operational flexibility and efficiency compared to having to set up closer airbases.

You don't have to set up a closer airbase.

You don't have to build it.
You don't have to conquer and secure that space (very costly).

Your airbases can be farther away from the front lines.

Immune from artillery and ground attack.
More layers of air defenses.
More warning of air attack.

You don't have to keep moving and rebuilding your airbases as the front line moves.

And all their fuel, ordinance, personnel, hangars, buildings, bunkers, defenses...
You can respond faster to changes on the ground providing better support.

You can strike much, much deeper into enemy territory.

The enemy has a wider area to defend, spreading themselves thinner.
Destroy their command and control and transportation, cripple the enemy, the war ends early, you save fuel, money, lives and suffering.

You can attack from unexpected directions.

Put a tanker where you have no airbases, attack from that direction.

But does it save fuel?
Central point number one: combat aircraft are inefficient, tankers are efficient and somebody has to drag that fuel out there.  Might as well be a tanker.
While keeping a tanker in the air might seem like a waste of fuel, and it does have to burn fuel while in the air, it likely saves a lot of fuel because tankers are so efficient and combat aircraft are so inefficient.  A tanker is typically based on a civilian airliner.  Once it's at altitude and in position, it can run racetrack laps at its most efficient speed and altitude (many combat aircraft have trouble going slow enough for the tanker) sipping fuel compared to the ordinance laden (ie. lots of drag), high performance aircraft it's fueling.
If that very efficient tanker didn't drag that fuel out there, those same laden, inefficient combat aircraft would have to and this leads to what is known as The Tyranny Of The Rocket Equation which says to carry more fuel you need even more fuel.  If you have a lot of inefficient aircraft, it can save fuel to have them take off with half empty tanks and top up at an efficient tanker on the way.  The Rocket Equation is so tyrannical you can save fuel even in civilian flights, here's a paper on the subject (sorry about the paywall).
Central point number two: make as few combat trips as possible to destroy a target.  A plane can only carry so much, and more fuel means less weapons.  Less weapons means more trips.  More trips means more fuel.  More importantly it means more times the pilot and airframe are exposed to danger (and if you need to take two trips you've probably lost the element of surprise), and more aircraft needed to attack the same number of targets.
Take off is another consideration.  Taking off and climbing to altitude is one of the most fuel hungry parts of a flight.  Combat and long haul aircraft typically take off near maximum weight requiring full engine power and a long climb.  Taking off with less fuel means you burn less fuel getting to altitude.
Heavier aircraft need a longer runway to take off from.  That runway length may not be available at your forward base, or it may be damaged.  Less fuel means heavier weapon loads with shorter runways which means more flexibility.
Combat aircraft can only carry so much fuel internally before they need drop tanks.  Drop tanks add drag making the aircraft less fuel efficient and generally maneuver like a pig.  Drop tanks also take up precious hard point space for weapons which means more round trips which means... you get the idea.
You can take this idea too far.  The biggest problem with the F/A-18 Hornet was its measly fuel load.  The long ranges typically required at sea meant it practically required a tanker to operate effectively.  Carriers can't carry big tankers, so they had to resort to inefficient buddy packs mounted on smaller aircraft.  This problem was resolved with the F/A-18 Super Hornet which is practically a different aircraft.

Technically, the purpose of inflight refuelling is to act as a force multiplier.
Say I have a requirement for a combat air patrol (two aircraft on station all the time) at a given location 250 miles away. The aircraft available have a range of 1000 miles and the ground turn-around takes the same time as flying 250 miles.
I need two in the air going there, two on station, two coming back and two on the ground - minimum of 8 of which 6 are not patrolling. Now put up a tanker and  even with only one refuel per patrol I can now keep between 2 and 4 aircraft on patrol at all times with only 4 aircraft.

Thinking of it from logical standpoint, we can define the exact set of circumstances in which it would be cost effective to do this.

The plane to be refueled is incapable of accomplishing its mission by going from any valid (i.e., reachable given time and distance constraints) launch spot to any valid landing spot;
and

The plane is capable of accomplishing its mission by going from any valid launch spot to a valid landing spot, if one or more refueling tankers can be got to some point along the route;
and

The tanker(s) can get to that point along the route;
and

The value of the mission is greater than the costs, even with the refuelling.

There are also cases where these rules don't necessarily have to hold. For example, if a plane can get to the target and back on its own power, but would have to fly a more dangerous route, the reduced risk of using a tanker may be worthwhile. At $2.4Bn per B-2 bomber, a mere 0.04% reduction in mission risk is worth over a million dollars in hardware alone: not counting costs from failing the mission, payload value, potential lost-pilot-rescue costs, new-pilot-training costs, political costs, and opportunity costs for being one plane down.

It can be a regular non-emergency military practice.
One major early example was US airstrikes on the Hanoi area during the Vietnam war, which were launched from far to the south, and would refuel just before crossing into North Vietnamese airspace, so that the planes would have as much fuel available as possible during the mission.
During air combat, particularly for aircraft who have missions that aren't just to go to one place and return immediately (e.g. to engage flying enemy aircraft), the amount of fuel carried (above what's needed to get there and back) determines the amount of time that aircraft can do tactically-useful things. Being able to top off closer to the target maximizes that time. For aircraft with afterburners, this also contributes to the amount of time they can use those, as using afterburners consumes fuel very quickly.

Lets say you're on a carrier and you want to bomb a target 200 miles inland.  What do you do?
Carriers normally stay a few hundred miles out to sea for safety--if they got too close they would be vulnerable to attack by Harpoonskis.  Oops, you don't have the fuel to reach your target with a load of bombs under combat conditions.  If you want to hit that target you're going to have to launch with tankers and you very well might need to meet tankers on the way back also.

A very common practice in the military is to take off with almost zero fuel and to refuel right after the initial climb to altitude, when operating fast movers from forward operating bases.
The practice occurs for fighter jets because their wings are designed for high speed, thus would require a very long runway to be able to carry their full weapons compliment into the air with their fuel. By sending up the fuel on cargo style planes, they can overall reduce the runway length required to fly fighters with a full combat load out (as the cargo planes are designed to lift things).
Once in the air the fighters are moving much faster, so they can carry the extra weight easily. The extra weight in the combat load out was decided to be fuel simply because it is easier to refuel in mid air than it is to rearm in midair (missiles cannot be easily piped).

Fuel is cheap.
Some more valuable resources that may be rendered useless or vulnerable during refueling include combat aircraft (which cost orders of magnitude more than refueling planes), nuclear missiles, training combat pilots or presidents.

You either fly the airborne plane from it's current location to land/refuel and back to the previous current location, or make the refuelling plane do it.
Either way, the same journey is being made.

Even if your fighter has to go half-way home to meet a tanker in the middle to refuel, that's half the journey it would otherwise have had to make. That's a huge saving in time and if you want your plane to be close to where it needs to be then you don't really have much choice. You can't have it spending half its mission going back and forth between home bases.


text:
Post license friendship - can club force instruction?
I have a situation that is a bit of a grey area and was hoping for some advice from other pilots that perhaps have been in the same situation.
After getting my license, my instructor and I stayed friends. Every now and again (maybe once a month or so), we go on pleasure flights together. We usually fly somewhere, have lunch, and then fly home.
Now I have a share in the plane we use and I pay for the time like I would any other time. I have always offered to pay him for instruction for these flights (even though it's not really), and he has always declined, instead allowing me to pay for lunch.
However, one of the club owners heard about these days out and now insists that this is recorded and paid for as instruction.
I have two issues with this: 1 - it doubles the cost per hour. 2 - I don't get my P1 hours up in my logbook. The cost I don't mind as much... I just want to build my solo hours.
I must re-iterate that I do all of the flying, although we do talk flying the entire time usually and he of course gives me additional insight and tips etc.
I'm not sure who is "in the right" here. The club IS a business, which I appreciate. But does that mean that an ex-student and flight instructor have to pay to be friends after training?
For reference, this is in the far North of the UK.

The whole "Not logging P1 time if there's an instructor on board" I wouldn't worry about. If you're the sole manipulator of the controls, sitting in the LH seat, and your licensed to fly that aircraft, you're P1. Just make sure the guy sitting next to you agrees.

Just thought of the one question I should have asked before answering: When this person is our pleasure flying with you, are they "on the clock" (ie employed by) the club? Or is this in their spare time? I think you probably know where I'm going with this...

@Cloud As I also trained in the 'far North' of the UK (well, "far" from a point of view of Londoners, otherwise knows as North East England), I'm curious if our paths may have crosses as there's aren't too many flying schools (or airfields) in that part of the country.  I suspect you are avoiding naming and shaming, though.

I think this can only be answered by reference to the specific bylaws of your flying club.  (Is the club organized as a business, or non profit?  Has the club owner referenced terms of a contract you agreed to?  How is the "club" any different from just renting?)

@Jamiec Spare time, on a day no instruction is held... :)

@MickWaites I am being intentionally ambiguous, I don't want to talk specific clubs :)

If you're just two friends having a pleasure flight, why are you buying him lunch? If you took me on a pleasure flight I think buying lunch for you would be the least I could do.

I think you just have to confirm with the instructor that he's not "on duty" when flying with you, will not be logging it, and you will log it as P1. That's it that's all. In the glider club world, we avoid this problem because A: you fly with all of the club instructors, whomever's on duty, and B: the instructor works for free anyway in a non-profit club.

Does the plane belong to the club and is it for training use only?  Is your former instructor employed solely by the club and was he 'on the clock'?  Only if one of these is true should the club have say over this.

Similar to Monty's comment: Is your airplane ownership share through the club? Bylaws would tell what you can do with your share. Is your instructor friend employed by the club? The employment contract would tell what he can do outside his employment.

I had a very similar situation, although without a club involved.
The chap who taught me to fly through my local flying school in the early 2000's was the same guy I got back in touch with in 2016 when I wanted to go back to flying after about a 10 year hiatus. By this time he had gone independent, but had use of a few aircraft at my local field and got me back to the point where I could redo my skills test and get my license valid again. We became pretty good friends socially, and as he had some contacts with aircraft I could rent I more often than not flew with him.
I always paid him for his time. Not least of which because that was his job and time spent flying with me was time he wasn't spending with students. For the record, he usually bought me lunch!
After about 3 years of this I realised something important. He was a crutch, and it was stopping me being fully confident. Having a friendly flying instructor next to me meant that I wasn't making the decisions which I should have been. It wasn't that he was doing anything wrong, but you often learn more from making a small mistake yourself and fixing it than someone with miles of head space pointing out the mistake you're about to make before you make it.
I've said it before here, I'll say it again - the best thing I ever did was started flying with other pilots (not instructors).

This is pretty much a legal question: about aviation law & regulation, and the flight club's right to dictate matters of this nature.
I'm not a specialist in UK aviation law, hence I advise you to find out (possibly by asking about it here) whether buying dinner to your FI companion could be considered compensation, possibly altering the assumed recreational nature of the flight.
This can turn out to be a tricky matter to resolve, and I suspect this is the reason your club owner is requiring you to log this as instructed time: he simply wants to make sure that should anything happen, there is no chance for having anything the regulators can sink their teeth into. Even if nothing out of the ordinary ever happens, keep in mind that there most probably are some routine inspections into the club log-keeping, and having you regularly fly with an instructor without the flight being logged as instruction might raise eyebrows.
And: I do not wish to imply that there is any foul play on your instructor friend's part, but the thing is he cannot log the time flown with you in any way, if you in fact are the PIC.
On the other hand, the club has very limited possibilities to legally limit whoever you should wish to fly with as long as your relationship to the person on board is purely casual when it comes to context of aviation. In this case there is no reason for them to enforce any logging practices but I reiterate, the case in question understandably can be seen as problematic from club's point of view.
As for JamieC's answer, I must agree. Unless the instructor is actually a friend of yours, "acting in that capacity", and the flights with him are relatively few and far between it is possible that they actually are counterproductive. One thing I loved about aviation is that you are on your own up there; I alone was responsible for managing the situations. That was the whole point of it, to a large extent at least. But I digress...

A good place to start would be to have a look at both the agreement between you and the club and the agreement between the instructor and the club to see if there are any terms about that.


text:
When was the last collision in IMC outside controlled airspace?
Following on from this question and the answer mentioning Big Sky Theory: When was the last collision in IMC outside controlled airspace?   I'm interested in finding empirical evidence for Big Sky Theory in class-G and IMC.
Most collisions seem to occur in VMC (VFR or IFR) or in controlled airspace although it's difficult to get the exact details from the descriptions online.  There are some reports on Pprune, but many incidents have one-line write-ups that don't mention the meteorological conditions of the airspace classification.

Around the world or in a specific country?

@Ben  Anywhere.  I'm interested in empirical evidence for Big Sky Theory in class-G and IMC.  Good point, I'll edit the question.

This sort of accident happening rarely might mean that the activity is pretty safe (i.e. big sky theory works), or it might mean that the activity, while not particular safe, is so rare that the accident rate is low despite the risks. Most places somebody would need to fly IMC for any great length of time tend to be controlled airspace, and the airspace that the airplanes that fly the most (time, miles) is Class A because that's where the efficiency is to be found. "Big Sky Theory" is only tested when 2 aircraft are somewhat close to start with, and that seems maybe rare in IMC + Class G.

In February 2020, there was a mid-air collision involving two IFR aircraft on training flights, in class G airspace in Australia. It occurred near a small airfield north of Melbourne. You can read the investigation report here. Essentially, while ATC did provide traffic advice to each aircraft, the pilots did not recognise the risk and so did not coordinate over the CTAF.

+1  From that report the collision occurred at 4,100ft and the conditions were broken with a base of about 3,200 ft amsl - I think that's within the ICAO definition of IMC - and the collision occurred in class G.

@Dave, note that "IMC" simply means the pilot is unable to fly at their current altitude solely by visual reference outside the aircraft. The ceilings can be 10,000' above the ground (so "VFR" at the reporting station) and an aircraft flying at 11,000' above the ground could be IMC within that cloud layer.


text:
Are there any TLS approaches in the US? What does a TLS approach plate look like?
I've been reading about Transponder Landing System (TLS) approaches: https://www.anpc.com/instrument-landing-systems/
What would a TLS approach plate look like? I have ForeFlight and can also get stuff off the FAA website. However, I do not know what airports to look at for approaches. I did try looking at Columbus Gorge Regional Airport (KDLS) as I have seen ANPC saying that airport has their equipment at it.
Does anyone know of any US airport that has a published TLS approach that I can look at for a plate?

Made a couple changes for grammar and acronym expansion, but have no idea what APNC is.

https://www.anpc.com/ is Advanced Navigation & Positioning Corporation.

Here is a current TLS approach chart.

It’s for the US base and is flight checked by the FAA.

In 2011, the only TLS in the US was a test system in Atlantic City:

The only civilian TLS in the United States is a Federal Aviation Administration (FAA) test system at the agency’s technical center in Atlantic City, New Jersey. However, although the FAA in 1998 certified the TLS as at least meeting ICAO standards for Category I ILS signals and in 2001 granted type acceptance to ANPC’s TLS, the systems are not likely to be widely used in the United States.
In recent years, the FAA has instead emphasized the development of instrument approach procedures using the wide area augmentation system (WAAS), a space-based navigation system with a ground-based network of reference stations and master stations that the agency says will not only enhance safety by adding precision-like approach capability but also eliminate the need for installation and maintenance of local airport-based approach equipment.

(flightsafety.org, emphasis mine)
I couldn't find any current civilian TLS approach in the US, so most likely none were installed. The approach chart would however look like an ILS chart:

Pilots fly a TLS approach just as they would an approach using an ILS, ANPC says, so “to the pilot, there is virtually no difference.” For example, TLS approach charts look like ILS approach charts; a failure of ground-based equipment to provide lateral or vertical guidance results in a red flag on a cockpit instrument, as it would in an ILS or other instrument approach; and a TLS approach always includes a missed approach procedure — although the TLS itself does not provide missed-approach guidance.

(flightsafety.org, emphasis mine)
I found an old picture of the TLS chart at Atlantic City (but I cannot find a current one, maybe it was removed there as well):

(image source: AFMAN11-217v1 on GitBooks)


text:
Did pilots use the radio in the Spanish Civil War?
Was radio communication used by fighter pilots during the Spanish Civil War?
Fiat CR32, Messerschmitt Me 109 and Polikarpov I-16 were typical aircraft but I have no information about radio communication during dogfights. Can anyone help?

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.

The Heinkel 51s used in the Spanish Civil War had radio equipment.  Messerschmidt Bf 109 versions A to E served in the conflict, the model B1 had FuG 7 radios.  I can't find a source but I'd guess that the 109 a had a FuG 2.
The Fiat CR 32 had an RA 80-1 radio as an optional piece of equipment.
The Polikarpov I-16 was introduced into service in 1933 without a radio, but by 1939 one had been fitted.
During the conflict, Werner Mölders developed a combat system that relied on radio co-ordination, there's an interesting article about that here.

Thank you Dave. Very informative.


text:
Are aerial tankers converted aircraft or their own designs?

For instance, this KC-46. Is this a converted Boeing 767? Or its own design?
Same with this Rivet Joint.

Because of the cost of designing, producing and then maintaining another aircraft type, it's just easier to start with the existing aircraft project, especially one produced in large numbers already.

Some tankers (DC-9s that I know of, possibly others) only use the 'standard' fuel tanks and don't have any additional capacity built in, as this is far easier from a certification point of view.

It depends on what you mean by converted. The design is converted from an existing model but most of the time they do not take a pre-built plane and physically change it.

The tankers are "based on", not a regular passenger jet "converted to".
The same engines and basic body, but the whole internals are different.
The Wiki page for the KC-46 outlines this
https://en.wikipedia.org/wiki/Boeing_KC-46_Pegasus
Some, but not all, of the other platforms are ex-airliners.
https://www.air-and-space.com/Boeing%20707%20Military.htm

"EC-18B, 81-0894 at Vandenberg Air Force Base on June 9, 1990. It
carries Boeing construction number 19583 and was delivered to American
Airlines as 707-323C, N8403 on November 28, 1967. The Air Force
acquired it on July 15, 1982. "

The E-3B AWACS platforms were purpose built by Boeing.

Would this be the same for the Eg18 Growler?

The fact that none of the C-135s show up on the military 707 page indicates they are totally separate designs.

I don’t believe an EA-18G Growler is used for tanker duties.  The F/A-18E/F aircraft could perform tanker duties but used an external drogue pack on the centerline hardpoint for that.  The fuel system is internally plumbed to allow gas to be exchanged that way.

not necessarily.  For example the Dutch Air Force used to operate several KDC-10As, which were converted DC-10-30s purchased from Martinair when they retired the type. Other countries operate converted 707s and no doubt others as well.

The Airbus A330 MRTT (Multi Role Tanker Transport) is actually built on the same assembly line as the regular passenger A330s in Toulouse and then flown to Spain for modification:

The A330 MRTTs are based on Airbus’ popular A330 widebody passenger airliner, with the aircraft produced on the company’s commercial airplane final assembly line in Toulouse, France. Once their initial built-up is complete, they are flown to Airbus’ military aircraft facility in Getafe, Spain to be transformed with hardware and systems for their dual roles as an air-to-air refuelling platform and an airlifter for troops and cargo. [...]
During the conversion, Airbus teams install some 16,000 types of new components and approximately 450 new electrical harnesses (for a total cabling length of more than 50 km.), as well as 6,000 brackets and 1,700 connectors.

(airbus.com - Airbus transforms A330s into Multi Role Tanker Transports through an optimised conversion process)
The cabin can be configured in different ways, similar to how a passenger A330 could be configured based on the customer's need. The cargo hold is also available with the same ULD containers as the passenger variant:

Serving three missions with one capable platform, the A330 MRTT can carry up to 111 tonnes of fuel in the aerial refuelling mission – the highest capacity of any tanker aircraft. [...]
Requiring no reconfiguration, the A330 MRTT also serves as an efficient transport aircraft, carrying passengers on the main deck and cargo in the lower lobe. Seating capacity is up to 300 passengers on the main deck, which also can be arranged in a medical evacuation configuration that accommodates 40 stretchers and 20 seats for medical staff, along with 100 passengers. The lower lobe – which accommodates up to 37 tonnes of cargo, can be loaded with 27 LD3 containers or eight military pallets.

(airbus.com - Multirole Tanker - Defence History)
Such a mixed configuration might look something like this:

(airbus.com - A330 MRTT)

That "mixed configuration" image at the end must refuel other aircraft from the MRTT's own wing tanks. I don't see anything in the cutaway that looks like extra fuel tanks...

@FreeMan I don't think any of them have extra fuel tanks. The normal fuel capacity of wing and center tanks is probably enough. Related: Do tanker aircraft distribute their own fuel?

You know, I probably read that answer when it was posted and 5 or 6 times since, but it just didn't stick. Thanks for the reminder.

I agree with WPNSGuy that most tankers are derivatives of transports, sometimes also bombers. But there are exceptions for sure.

The Dash 80 prototype led to the commercial 707 and the military
KC-135 tanker. Both planes shared the basic design of the Dash 80 but
were very different airplanes, neither one being a derivative of the
other. One great difference was in the width and length of the
fuselage. Airlines wanted the 707 fuselage to be 4 inches (2.5
centimeters) wider than the tanker’s.....

Source: https://www.boeing.com/history/products/707.page
So KC-135 and 707 are parallel but separate designs. In fact, military version of 707 is designated as C-137 or C-18 instead of C-135 and Boeing's internal designation of KC-135 would be Model 717

And in fact, the KC-135 carried the Boeing designation of “717” — certainly different than the Boeing 717 of today. I wish I had a picture of it, but the info plate as you ascend the crew ladder of the KC-135 states that: “Boeing 717”.

4"=2.5cm? Maybe...

@RetiredATC 4" is about 11cm

@jwenting I was referring to that conversation in the answer above.

One counter-example : KC-390 (Embraer) doesn't seem based on an existing airframe, but designed concurrently as tanker and/or military transport from the ground up.


text:
What do the 3-character aircraft type codes mean?
On some aircraft lists, including Eurocontrol's Aircraft Performance Database, aircraft have a 3 character code as their type. For example, an Airbus A320neo has a type of "L2J", a Boeing 747-400 has a type of "L4J" and a Cessna 152 has a type of "L1P".
I'm guessing the 2nd character is the number of engines (although some aircraft have a C for the 2nd character, and I'm not sure what that means), and the 3rd character is the type of engine (e.g. P for Piston, J for jet and T for turbine).
I'm not sure what the first character represents.
There are 6 options for the first character: L, S, A, H, G and T.
Does anyone know how these codes work?


text:
Why do Presidential flights have special call signs?
"One of the most powerful people in the world is on this airplane" does not seem like something you would want to broadcast, but "Air Force One" is effectively a code-word for that sentence. Why is it used? I would think that presidential flights would have inconspicuous call signs intended to attract as little attention as possible; maybe even fake airline call signs.

It wouldn't be called "Air Force One". It would be, if anything, something like SAM12345. "Special Air Mission". And SAM12345 could be carrying anyone.

@WPNSGuy so the President's plane's call sign isn't really Air Force One?

Correct. Not anywhere public with air traffic control, anyway. AF1, Or Navy 1, Army 1, whatever...would be the name of the aircraft. But not the callsign used by ATC.

@WPNSGuy, that's simply not correct... 7110.65 2–4–20a7

@someone yeah they have a fleet of planes with various tail numbers.  It's SAM .

@WPNSGuy https://aviation.stackexchange.com/questions/97482/is-air-force-one-used-as-an-atc-call-sign

The radio-telephony for the President flying aboard a USAF aircraft is, "Air Force One."

It would want to draw as little attention to itself as possible if it didn't have the full might of the military to protect it, if needed.

From Air Force One:

Columbine II is the first plane to bear the call sign Air Force One. This designation for the US Air Force aircraft carrying the incumbent president was established after an incident in 1953, when Eastern Air Lines 8610, a commercial flight, crossed paths with Air Force 8610, which was carrying President Eisenhower. Initially used informally, the designation became official in 1962.

So the original purpose of the "Air Force One" call sign was to distinguish the special flight from any other flight.

It's also worth noting that if the President is flying on a aircraft owned by the Navy, Army, or the USMC, the aircraft is referred to as Navy One, Army One, and Marine One respectively

@user253751 Since 1953 there has been substantial improvement in standardizing radio phraseology to avoid callsign confusion (as well as procedural changes to how the President flies). It ain't perfect - https://www.youtube.com/watch?v=b26NcJCLZl4 is an example of similar callsigns causing an issue - but AF1 on today's computerized systems is gonna be a) clearly labeled on the controller's scope and b) flying with a TFR that forbids other aircraft from getting anywhere near. Eastern Air Lines 8610 today would discover an F-16 gesturing angrily in their direction rapidly.

@ceejayoz So it may not be as technically important these days, but we keep doing it out of tradition (which has been codified into law).

@BryBuriya But the only other one of those that's in common usage today is Marine One, usually a helicopter.  Navy One has happened exactly once, when G.W. Bush landed on the USS Abraham Lincoln in 2003.  Army One has not been used since 1976.  Coast Guard One and Space Force One have never been used.  Executive One is used for civilian planes carrying the President - generally its only use has been for outgoing Presidents leaving the capital at the end of their term.

@ceejayoz I don't believe AF1 gets any special increased separation service while airborne, though I may be mistaken. They certainly don't get a moving TFR. (They certainly do get very meticulous traffic calls.)

It's difficult to be inconspicuous when you'll also want all other aircraft to keep far away from AF1. Besides that, it's public information what countries the president is visiting. The only special case I can imagine is if the president made one visit to Ukraine. In this case they'll figure out the best way of not broadcasting any security relevant information in advance. Some world leaders have done it.

Specific example: https://www.washingtonpost.com/archive/politics/2003/11/28/inside-bushs-top-secret-trip/4f0c7c94-6899-4ec1-961c-6e5eb012eb15/ - Air Force one used a false callsign, press embargoed, no TFRs.

And on the plus side usually using the call sign makes it a little bit harder for everyone else to spot the fake one


text:
What is the difference between an aircraft and a projectile?
18 U.S. Code § 31(a)(1) says,

The term “aircraft” means a civil, military, or public contrivance invented, used, or designed to navigate, fly, or travel in the air.

What exactly is an aircraft? Airplanes, helicopters, sailplanes, drones, etc are obviously aircraft, and I am quite certain that bullets and arrows are not aircraft (although I guess they do technically meet that definition?). What about something like a paper airplane, that does glide using aerodynamic effects but is not controlled or propelled after launch? Or what about an RC glider launched with a bungee (technically not very different from an arrow, except it's controllable)?

And CFR 1.1 defines it as: "Aircraft means a device that is used or intended to be used for flight in the air."  How many other definitions would you like to see?  And for what purpose or context?

@MichaelHall maybe legal precedent or FAA interpretations covering borderline cases?

The line is blurred, because every new invention may fall in one or the other way. Common sense will rule, until authorities will explicit tag stuff. So, why a paper plane is an "aircraft" (designed to fly), but not your lost umbrella (because of high winds)? (Exhausts are designed to travel in the air). Do no try to interpret laws like an engineer document (but also the contrary: there are a lot of interpretation and implicit conventions).

In what context do you wish to know the answer? A "Model Aircraft" for example may or may not be classed as an aircraft, in different contexts.

Oh yay, an argument about definitions.

@MikeB primarily for the purpose of determining whether the FARs apply to something's flight

I'm reminded of a case where a UK motorist was charged with having lights on his car that weren't working and he claimed they weren't lights, just ornaments, because he only attached them for decoration and never wired them up.

The difference is where it lands.

@Mark so if you shoot a bullet at a runway, that makes it an aircraft? :)

The technical difference is either it's lifted by air or not.

Federal criminal code aside, the definition of an aircraft under Title 14 CFR §1.1 defines an aircraft as “a device that is used or intended to be used for flight in the air”.  Flight, in this context, refers to sustained flight, either by means of air moving around lifting surfaces for heavier than air aircraft, or suspended from an envelope containing a gas, which displaces a volume of air weighing more than the gas contained within the envelope.  Projectiles, on the other hand, do not make use of sustained flight.  They are only capable of making limited air travel along a ballistic trajectory under the influence of gravity e.g. flight of an arrow or javelin.

{ I would have thought a cruise missile is not a projectile and is specifically not referred to as such? }

Interesting.  So would the Space Shuttle be considered a projectile?

There are cases of overlap here.  By definition, an aircraft is an object that uses air for support against gravity. This would include lifting mechanisms such as wings or buoyancy, and generally infers sustained flight.
A projectile is an object that has force applied to it to get airborne.  A typical example is a thrown stone or a cannon ball.
Gliders and rockets are interesting as they can fit both definitions.
Gliders are launched with external force yet have wings and are capable of sustained flight.  Rockets may or may not use wings.  While under  power they can be considered aircraft (or spacecraft), once fuel is expended they are projectiles.

And then you have things like submarine launched https://en.wikipedia.org/wiki/Tomahawk_(missile) It is initially launched by a bust of compressed gas (projectile), then it launches its rocket engine which powers it, as it transitions into starting the jet engine and unfolding its wings, (aircraft).

"once fuel is expended they are projectiles" - the same can be said about most non-glider aircraft...

Well there you go.  And what about controllable projectiles?  Seems there are 2 schools of thought:  powered and/or controllable.  And lift can be borderline too, as anything with an angle of attack will be lifting.

So how would you classify a bird in this definition?  (Or a bat, flying insect, etc.)  It does not have any force applied to it to get airborne, a bird flies under its own power.  It does operate on fuel (whatever food it eats).  Obviously they are animals rather than manmade objects, but let's say someone made a mechanical bird that flies in the same way by flapping wings (I've seen toys that do this using stored power in a spring or rubber band for example.)  The lack of control may be a defining factor in the case of toys...

By this definition, was the returning Space Shuttle a glider or a projectile?  It was under control, but unable to maintain horizontal flight (other than at the expense of velocity)    Nominally out of fuel it could only descend.

Aircraft implies man made, controllable.  @Criggie we can see how a glider can be considered both.


text:
Which models of the Boeing 737 are still in production?
The BBC is reporting that the production of 737 aircraft has been reduced from 52 to 42 per month due to a loss of 737-Max orders.  It isn't clear in the report whether the Max is the only 737 being made now.
Does Boeing currently produce any other models of the 737 or is the Max the only one?

More than half the models that ever existed are still in production.

By the way, neither Boeing nor the BBC article said that the production cut is because of a drop in orders. The most recent order numbers on Boeing's website are actually higher than they were in February. Boeing's statement said that the production cut was temporary and was to allow them "to prioritize additional resources to focus on software certification and returning the MAX to flight."

@reirab  Do you think that Boeing's statement is accurate?  They have about 5,000 unfulfilled orders and you can't really move aircraft fabricators to software certification?  Although, the only definite cancellations to date are the 49 Garuda aircraft.  There's an interesting article about the difficulties of cancelling here: https://www.businessinsider.com/boeing-737-max-8-orders-unlikely-canceled-experts-2019-3?r=US&IR=T   But I guess the customers could change to other 737 models???

@DaveGremlin It's unlikely that they would change to NGs. Operators don't spook as easily as the news media. They understand that they're talking about a potentially 20+ year investment. It's similarly unlikely that many will cancel unless something changes dramatically for the worst in the next few months. I would guess it's engineering resources that Boeing needed to move around for a while. With something as complicated as airliner production, those are still required. At any rate, significant order cancellation is completely unsubstantiated (and unlikely) speculation at this point.

By contrast, here are the models that are no longer in production.

737-100 Original (93' long, 85-118 seats) not replaced
737-200 Original (100' long, 102-130 seats) replaced by -500 Classic
737-300 Classic (110' long, 126-149 seats) replaced by -700 NG
737-400 Classic (120' long, 147-168 seats) see longer -800 NG
737-500 Classic (short 102' long, 110-132 seats) replaced by -600 NG
737-600 Next Gen (short 103' long, 108-130 seats) not replaced

The 737-900 (non-ER) isn't in production either.


text:
What percentage of professional pilots end their career early for medical reasons?
As an amateur pilot I'm aware that I can lose my medical at some point and have to give up flying.  For a professional, losing a medical can end their career. I know several people who wouldn't even get a PPL with their medical history. Are their any figures on the number of CPLs/ATPLs that lose their livelihood for this reason?

Don't know the number but I do know that the fallout rate is much lower than it once was.  You can hold a Cat 3 and even Cat 1 with all sorts of medical conditions, like controlled hypertension, some kinds of arrhythmias,  that once grounded people, if you meet certain conditions and once you jump through a bunch of hoops.  Canada has even issued a Cat 1 to an airline pilot with Type 1 insulin dependent diabetes (with extremely strict monitoring requirements).  I knew a guy with a Cat 1 with mild Parkinsons.  He had to quit when the shaking got too bad to write.  Another with vision in 1 eye

Do you have a country in mind?

@Ben - No country in particular

@John K - I did read an AAIB report where an airline captain was PF and his prosthetic arm came loose - https://www.bbc.co.uk/news/uk-northern-ireland-28778728. In the UK there's Cat 2 for private flyers and Cat 1 for the pros for EASA licences. You can downgrade your licence from EASA to LAPL or NPPL and use a driver's licence level of medical sign-off but it limits the type of aircraft you can fly, which many older PPLs do. I've also known people jump though all the hoops (£££) and still have to stop flying

I can't think of a source for this data. The FAA has an airman database but it has nothing about medicals.

In the ultimate fullness of time, ALL pilots lose their certificate due to "death", which is a medical condition.

This information doesn't easily exist, I'm sure insurance companies have some figures but they don't seem to be public.
I'm just going to expand on Pondlife's answer with more data. In Australia the regulator CASA publishes all sorts of statistics, including the number of medical certificates issued and refused (page 164).
Australian professional pilots need a Class 1 medical certificate. In the 2017-18 financial year CASA issued 18,155 certificates and only refused 26, or 0.14%. Over the past 5 financial years, 0.22% of applications were refused, and the trend is decreasing.
Some caveats on this data:

Not all of this 0.22% were already professional pilots, in fact I would guess that a significant amount were prospective pilots seeing if they would pass before investing so much money in training.
These numbers only include those who actually applied for certification. If a pilot suffered a major health diagnosis they probably wouldn't bother to even apply.
Class 1 certificates are not just for airline pilots. GA pilots, instructors and so on are also included in this percentage.
CASA has a reputation of being a bit more restrictive than the US, but are more allowing on some particular conditions.

So the answer to your question is more than 0, but not enough for a person of ordinary health to be worried about, in my opinion.

I like numbers.  Numbers are good.  As you say, not all those refused would be career pilots, and of those refused I'd expect many to regain their Class 1 after treatment (for example; I new an ATPL who spent 12 months recovering from multiple fractures as a result of a car crash)

(US-based answer)
The problem with answering your question is that it isn't all clear what "losing a medical" actually means. If a pilot is denied a medical, the FAA will obviously know and you could probably get that data from them somehow, even if it takes an FOIA request. But what if you just decide not to renew because you know you'd fail? For all the FAA knows, you might have retired, changed career or even died.
If we assume you do mean "denied" then the only specific number I could (quickly) find for denials was from 1978:

The annual denial rate based on airman applicants is 7.9 per 1,000
  airmen. By class of certificate applied for, the annual denial rate
  per 1,000 applicants is 4.5 for the first class, 6.0 for second
  class, and 10.2 for third class.

You could apply those rates to the number of airmen certified each year to get an estimate of the absolute numbers, but obviously using a 40 year-old report may not be accurate today. And determining how medicals 'map' to pilots may not be easy.
If you want a better answer, you'll have to define exactly what you're asking. To get an accurate number (whatever that means for your purposes) you might have to look for some specialized actuarial or medical research on this.

What I'm asking, exactly, is how many professional pilots' careers are cut short for medical reasons.  I'd like to know how precarious a profession it is.


text:
What is the correct formula to calculate propeller efficiency?
How to calculate in the right way the efficiency of a propeller?
If we know the engine power, speed of the plane and the thrust of its propeller, what is the correct method, (1) or (2), for calculating the efficiency of the propeller?

Assuming that Method 1 is the correct one, it appears that the efficiency of a propeller must satisfy the inequality:

Update:
It looks like Method 1 is correct and 2 is wrong as long as the page Performance of Propellers, MIT calculates the efficiency on an ideal propeller and gets the same inequality.
If a propeller of diameter, d, delivers the thrust, T, while the plane travels at the speed, V, it always has a maximum possible efficiency that can be calculated, is below 1 and can not be improved. Not even an ideal propeller of diameter, d, has, in general, an efficiency that reaches 100%. The absolute minimum, reference power, is always $TV$ and not something else and the efficiency is always:
$$TV/Power_{absorbed}$$
where the minimum absorbed power is calculated with Froude's Propeller Theory.

Propellers are designed for an optimal tip speed being a certain multiple of the air speed they are flying inside. The angle of the propellers aerofoil shape to the propellers axel changes as you move from the Center to the tip. The more pronounced the blade rotation the higher the intended tip speed.
From memory having the tip speed running at 1-3 times the the air flow speed is not efficient, but at least the propeller works over a wide range of air speeds.
Highly tuned propeller tips are moving at 6x the air speed. However, their effectiveness at moving air disappears rapidly if the air flow is too slow. (The propeller blades are stalled)
The last thing you want on a propeller plane is an efficient propeller.

I wasn't really able to follow the hyperlink.  For one thing, it seems to suggest that the propeller efficiency just before takeoff is zero.  I find that approach worthless.
Usually, I have found that if I need to know for sure if something it correct, at some point I will have to derive it myself, so it will usually take less time to do that then to search for what will be a non-authoritative answer anyway.
I would think a better way to handle this question would be purely from an input and output perspective, not a microscopic blade perspective.  It shouldn't matter if there are 10 blades shaped like a Christmas tree.  The only things you need to measure for efficiency ratios should be inputs and outputs.  That's the whole idea behind such numbers.
Here, the input is power.  The output is clearly thrust.  So, a propeller assembly's efficiency should be the actual thrust as compared to a perfect magical conversion of power to thrust in which there was no wasted energy going elsewhere and not into thrust.
These ideas in mind, we start with $$T_0={d\over dt}mv=\dot{m}\Delta v,$$ where $T_0$ is the thrust of a perfect fan assembly, $m$ is the mass of the fluid passing through the swept area of the fan assembly, and $\Delta v=v_{\rm out}-v_{\rm plane}$ is the average difference in speed between the fluid exiting the fan and rate that the fan itself is moving through the fluid in the rest frame of the fluid.
The use of the word average here is intended to allow for a "spherical chicken," such that the effects of turbulent flow outside of the fan swept area affects the efficiency, but can cleverly be ignored in our actual calculations of efficiencies themselves.  However, one could visualize the fan assembly to be a ducted fan with a constant cross-sectional area, where the air aft of the assembly could be of a higher mass density than the partial relative vacuum fore of it, just as in some automobile air intakes at high enough rpm. This way it is clear that the exhaust velocity isn't any speeds of airstreams near blades, which could be higher, but, rather, is the average velocity of all air just after the fan assembly.
Continuing, in the reference frame of the plane we have an increased kinetic power of the fluid due to the fan of $$P={\dot{m}\over2}(v_{\rm out}^2-v_{\rm plane}^2).$$ Mass flux analysis yields $$\dot{m}=\rho A v_{\rm out}$$ where $\rho$ is the mass density of the exhaust fluid excluding any fuels.  This gives
$$P={\rho A v_{\rm out}\over 2}(v^2_{\rm out}-v^2_{\rm plane}).$$
This is a cubic equation.  They generally have one real solution and a few imaginary ones.  However, we can use a trick from relativity, with $\beta\equiv v_{\rm plane}/v_{\rm out}$, yielding $$v_{\rm out}=\left[{2P\over \rho A(1-\beta^2)}\right]^{1/3}$$ and an ideal thrust of $$T_0=\rho A v_{\rm out}(v_{\rm out}-v_{\rm plane})$$
$$=\left[4P^2\rho A{1-\beta\over(1+\beta)^2}\right]^{1/3}.$$ Using $A=\pi(D/2)^2$, this is $$T_0=\left[\pi \rho P^2 D^2{1-\beta\over(1+\beta)^2}\right]^{1/3}$$ with $\eta_{\rm P}=T/T_0$ where $T$ is the observed thrust of a fan assembly using a power $P$.
The dependence upon $\beta$ alone, as if the power were fixed, is

I think this is "How to calculate in the right way the efficiency of a propeller."
To check if this analysis is reasonable, we can compute the efficiency of one of the first propellers. I did that in the answer to Why does this calculation show Gustave Whitehead's propellers were more than 100% efficient? and arrived at 81±13%, which seems to me to be a reasonable efficiency.

The zero efficiency might seem worthless indeed, but uses an approach which is not valid for the static case. It produces a simplified formula which works reasonably well for flight conditions. How the static formula is derived can be found here